In the design of large buildings and other structures for resistance to earthquakes, a recent practice is to isolate the base of the structure from its foundation by means of numerous large rubber or other elastomeric pads. Such structures are called base-isolated structures. In one example, a large steel-frame building has about 100 columns rising from a rigid planar base. The base is supported about 60 cm above a foundation slab on large rubber pads about 60 cm thick and 120 cm square; there are four pads positioned below the base of each column.
In an earthquake, the pads act as "shock mounts" or "vibration isolators", reducing the peak accelerations of the base of the structure compared to the corresponding accelerations of the foundation. Sudden displacements of the earth are absorbed or stored by the pads, then retransmitted more slowly to the base. During this process, there are cyclic relative displacements between the foundation and the base.
It is useful to have a record of such relative displacements after an earthquake. Relative horizontal displacements are known to be more significant than vertical. Prior recording practice is to install accelerographs in structures, then integrate the recorded data twice with respect to time to obtain displacement data. Relative displacement data require taking the difference between double-integrated acceleration data from two accelerographs, instant by instant; the process is laborious and prone to error.
It is hence desirable to provide an instrument that will record relative displacement directly.